The trend to thinner and thinner mobile phones, as well as to increasing resolutions, leads to lens modules with particular designs; a lens module being constituted by an assembly of one to several lenses and diaphragms into a lens holder. Also, for modules aiming at large volumes markets a particular attention must be laid on the manufacturability, because a production of several tens of thousands of lens modules per day can be envisaged only when the manufacturing yield is close to 100%.
Depending on the customer's specifications, the constraints that have a major influence on the design are:
Effective Focal Length.
The Effective Focal Length determines the overall dimension of the module. The Effective Focal Length will hereafter be referred to as EFL.
Back Focal Length.
The Back Focal Length, or BFL, is the distance between the apex of the lens closest to the sensor, which is the intersection of the optical axis with the surface of the lens closest to the sensor, and the top surface of the sensor.
Convergence.
The Convergence C is the inverse of the EFL. It is also called the power of the lens.
Field of View
Combined with the dimension of the image that will be formed on the focal plane of the module, the EFL determines also the Field of View, referred to as FOV.
When a circular image of diameter D, centered at the intersection of the optical axis with the focal plane is formed in the focal plane of the module, the FOV is defined by the relation:FOV=2. Arctan(D/[2.EFL])
where Arctan is the inverse function of the tangent of an angle.
When the image is rectangular, this diameter D is the dimension of the diagonal of the rectangle. The center of the rectangle is at the intersection of the optical axis with the image plane, and is referred to as the image center.
It is also a common practice for defining the position of a point to use either the image height, which is the distance of this point from the image center, or the angle which subtends the segment between the image center and this point, both expressed as a percentage “x” of the maximum half FOV, and noted x % hFOV.
It is well known in the industry that the larger the FOV, the larger the geometric aberrations, and the more difficult it is to realize a lens with high resolution and low astigmatism.
Aperture Number F#
The Aperture diaphragm of the module which limits the light entering the module, and combined with the EFL determines the Aperture Number of the module, referred to as F#, equal to the ratio of the EFL by the diameter of the Aperture diaphragm.
The aperture diaphragm is usually called Stop, as it functions to limit the light into the module.
The F# has a major influence on four important parameters: the light reaching the sensor, which varies as the inverse of the square of the F#, the Depth of Field (DoF), the Hyperfocal distance (HyF) which is the point of focus where an object from half that distance to infinity gives a neat image on the sensor, and the depth of focus (dof) which is the tolerance on the position of the sensor with respect to the lens module.
Resolution
The resolution is measured by the Modulation Transfer Function (MTF) at a given spatial frequency. The resolution characterizes the contrast between the black area and the white area in an image composed of a series of alternatively black and white stripes of equal width, the width of a pair of stripes being the inverse of the spatial frequency.
The MTF is usually expressed as a percentage of the maximum possible contrast. The maximum MTF is 100%, and the MTF performances of a given lens vary from 0% to 100%.
The spatial frequency is expressed in Line Pair Per Millimeters, abbreviated in 1 ppm.
Aberrations
The aberrations are both geometric and chromatic.
The geometric aberrations include the geometric distortion, the astigmatism, and the EFL differences between various areas of the image. They depend on the curvature on axis of the lenses, and on the asphericity coefficients that define, at a given distance of the axis, the distance between the surface of a sphere having the same radius on axis and the surface of the lens.
The chromatic aberrations include the “colored fringes” (edges of an object are surrounded by parallel edges of various colors) and the “colored area” (a white image presents for example pink corners).
Targeted Costs
The targeted cost of the module depends primarily on the number of lenses comprised in the module. The present invention allows reducing the aberration by balancing the convergences of the lenses, rather than by adding more lenses.
More details on the EFL, C, BFL, FOV, MTF and F# can be found in the academic literature, for example “Modern Optical Engineering” by Warren J Smith, McGraw Hill.
With so many constraints, one can understand that a lens module is designed for a p articular set of specifications. However, as very often for a given sensor the phone makers develop several models with slightly different characteristics (slight variations on EFL, FOV, and MTF specifications). It is possible to design a module with some versatility by giving a range of variation to the various design parameters.
There exist a number of publications that describe modules composed of four lenses, or four groups of lenses, with restrictive conditions on the type of lens being used, the focal lengths of the lenses, or on the asphericity coefficients of the surfaces, which are tailored for particular applications and that differ from the present invention; for example: JP4479715; JP2003098428; JP2007108770; US5367405; JP7098430; JP1 1190820